Many young children get confused by some of the language regarding eating. The standard question "Are you full?", meaning 'did you get enough to eat?' often conjures up an image of being stuffed with food from the tip of their toes to the top of their heads.
A lot of kids seem beset with the idea that humans are empty sacks that must be filled up at every meal. Of course, they know nothing about biology. About how food is converted into energy that powers the body, and how that fuel must be regularly replenished. How that fuel is separated from waste products and how one is used while the other gets expelled.
And they certainly have no idea that anyone has anything as exotic as a circulatory system.
Of course, even if you were such a fanciful youngster, now, you surely know about plants and animals converting food into energy and the systems used to move nutrients from one point to another.
Still, ahead of your GCSE Biology exam, Superprof gives you an overview.
The Basics of Transportation
Every living organism needs food, water and oxygen; these ingredients, so vital to life are needed throughout the organism, not just one or two targeted areas. Consuming these products generates waste; the by-products the organism in question has no use for. These waste products must be excreted or expelled.
Autotrophs - plants, bacteria and algae do not need a food source; they produce their own food through photosynthesis. You'll remember that photosynthesis is the process of converting light energy into chemical energy which, then, through cellular respiration, fuels the plants' growth.
In this process, light acts on carbon dioxide and water, turning them into glucose and oxygen. As the plant uses very little oxygen, most of what it produces is considered a waste product, which the plant then expels. When excess water is present, it too becomes waste and must be expelled through stomatal pores.
Transport is much more sophisticated for animals. For one, oxygen, food and water have to reach every part of the body, which calls for an extensive circulatory system. And, just like plants, animals must expel waste products. Their excretory system, like the circulatory system, is much more complex than those of plants.
So, while there are some similarities between animal and plant nutrition and transport systems, the differences are great enough that each warrants its own segment.
Transport in Plants
Vascular tissues make up plant transport systems. The two primary types of tissues are phloem and xylem. They are arranged as vascular bundles throughout the plant with the phloem forming the outer layers that cradle the xylem.
Phloem is made up of living cells, arranged end-to-end and extending throughout the plant. Its purpose is to transport sucrose and amino acids from the leaves to wherever they're needed in the plant - in any direction, including up or down. Naturally, this action takes energy.
Phloem is made up of sieve elements, parenchyma, fibres and companion cells.
The sieve elements act as conducting cells; they carry the sucrose throughout the plant. When fully mature, they have no nucleus and only a few organelles; the companion cells supply almost all of their metabolic needs. These companion cells are a particular type of parenchyma cells; the other type is called albuminous cells.
Note that albuminous cells feature only in seedless vascular plants (ferns and certain types of mosses) because they are linked strictly to sieve cells. Other parenchyma cells remain undifferentiated; they can be used as food storage units.
A bit more detail on companion cells: they may be ordinary - with smooth walls and no attachment other than to sieve cells, or ridged-body transfer cells. Their job is to scrub for solutes in energy-driven cell walls.
Phloem takes on the task of translocation; moving food to different parts of the plant.
In plants, water transport flows in one direction: from the roots up. It happens by way of cohesion and adhesion, a system that requires no energy at all. This is the xylem's function. Besides providing water transport, it also conveys mineral salts throughout the plant.
Xylem cells are not living. They are arranged in a long, continuous formation, from the root hairs to the very furthest extremities, and do not leak any water along the way. You might think of them as enormously long drinking straws.
Xylem is made up of xylem parenchyma, tracheids, xylem fibres and vessels, and they occupy the centre of the vascular bundles.
Any excess water the plant may have is released in a process called transpiration, which mainly involves the xylem. These channels send water to the leaves, where it evaporates once expelled through the stomata.
Transport in Animals
For all animals' biological complexity, their transport also boils down to two parts: the circulatory and excretory systems.
Animal circulatory systems, comparable to plants' phloem because they deliver nutrients throughout the body in defiance of gravity, are closed-loop systems. In this key aspect, plant and animal systems differ.
The heart drives animals' circulatory system; the rest of it is made up of blood vessels and the medium containing nutrient-rich, oxygenated fluid - blood. That is, as it's leaving the heart.
As blood circulates throughout the body via the arteries, it distributes food and oxygen. Once through the capillary barrier - the smallest blood vessels, blood enters the veins, picking up carbon dioxide along the way.
And then, it's back to the heart's right ventricle, to be pumped for a loop around the lungs - another round of oxygenation and back to the heart's left ventricle, where freshly oxygenated blood is pumped through the body once again.
This is how things work for most animals but single-celled organisms - think: paramecium and the like, don't have such a complex circulatory system. These unicelled bodies absorb nutrients through diffusion.
Larger aquatic animals, like jellyfish and hydra, have a water vascular system through which they circulate nutrients.
The waste that accrues in animal bodies is ejected through their excretory system. It's made up of kidneys, ureters that convey waste to the bladder, and the urethra, through which waste leaves the body. This waste may comprise excess salt and blood sugar, carbon dioxide and other undigested waste.
Again, the amoeba, paramecia and other lower life forms stand out; they do not have an excretory system. Instead, they diffuse waste through their cell membranes.
You might wonder why solid waste systems are not counted in animals' excretory systems. It's because the colon is a part of the digestive system... which leads to a larger question: why isn't the digestive system considered transport? After all, it carries food along its path.
If you think of the digestive system as a factory to extract nutrients from food, the answer becomes clear. The digestive system is less a system of transport than it is an assembly line that breaks food down into components that can travel along with the body's transport systems.
Or, more accurately where digestion is concerned, a disassembly line.
There is some transport inherent to assembly lines but it's more of a start-stop proposition; not a continuous, ever-flowing affair.
Differences in Transport Systems
We now know that there are some similarities between plant and animal transport systems and quite a few differences. To cap things off, let's lay them out, side by side.
- plants have phloem and xylem for transport; they are not closed-loop circulatory systems
- animals have blood vessels and a pump (the heart) to drive their circulatory systems, and a separate system to excrete waste
- in plants, not all parts of the transport system include living cells
- animals' entire transport system is made up of living cells
- plants' transport systems convey sugar, water, minerals and amino acids
- animals' transport systems circulate vitamins, hormones, glucose and glycerol; minerals, fatty acids and amino acids
- plants' transport mechanism uses little energy; water and minerals transport requires none while translocation does
- animals' transport mechanisms are energy-intensive, particularly the heart pumping blood through its vessels
- in plants, transport is controlled by the stomatal activity
- animals' blood flow is driven by constriction (heart) and vasodilation (blood vessels)
- plants' energy consumption is limited to ATP stimulating transport in the phloem
- in animals, energy from ATP is required throughout the transport system
ATP stands for adenosine triphosphate, the energy-giving molecule used for all cellular activity in plants and animals. Any activity the transport system undertakes draws on the organism's ATP stores. But being an organism's energy source is not ATP's only function.
ATP is also a signalling molecule, the means by which cells communicate. Cellular communication might involve commands to grow, become a different type of cell, metabolize and even die.
Kinases are instrumental in this communications process; they are also great consumers of ATP. Kinases are enzymes that transfer a phosphate molecule to substrate molecules.
This is much too ponderous a subject for us to get into here. You should take a look at our in-depth explanation of enzymes to learn more about them.
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